Mass Variance from Archival X-ray Properties of Dark Energy Survey Year-1 Galaxy Clusters

Using archival X-ray observations and a log-normal population model, we estimate constraints on the intrinsic scatter in halo mass at fixed optical richness for a galaxy cluster sample identified in Dark Energy Survey Year-One (DES-Y1) data with the redMaPPer algorithm. We examine the scaling behavior of X-ray temperatures, T_X, with optical richness, λ_RM, for clusters in the redshift range 0.2 < z < 0.7. X-ray temperatures are obtained from Chandra and XMM observations for 58 and 110 redMaPPer systems, respectively. Despite non-uniform sky coverage, the T_X measurements are >50% complete for clusters with λ_RM > 130. Regression analysis on the two samples produces consistent posterior scaling parameters, from which we derive a combined constraint on the residual scatter, σ_{ln T | λ} = 0.275 ± 0.019. Joined with constraints for T_X scaling with halo mass from the Weighing the Giants program and richness–temperature covariance estimates from the LoCuSS sample, we derive the richness-conditioned scatter in mass, σ_{ln M | λ} = 0.30 ± 0.04 _(stat) ± 0.09 _(sys),, at an optical richness of approximately 100. Uncertainties in external parameters, particularly the slope and variance of the T_X–mass relation and the covariance of T_X and λ_RM at fixed mass, dominate the systematic error. The 95% confidence region from joint sample analysis is relatively broad, σ_{ln M | λ} ∈ [0.14, 0.55], or a factor ten in variance.